The catalytic reaction of an alkene oxide such as propylene oxide (PO) and carbon dioxide may results in various products, e.g cyclic propylene carbonates used as solvent in coatings. Another important product is polypropylenecarbonate, an alternating copolymer obtainable from the ring opened reaction of said alkene oxide (epoxide) in the presence of carbon dioxide.
Polyalkylene carbonates are useful materials because of their properties such as clean combustion, transparency, excellent gas barrier, improvements of tensile strength in blends etc. These polymers may be obtained from the alternating copolymerization of carbon dioxide and oxiranes (epoxides). The process needs a catalyst in order to proceed. These catalysts may either be molecular and soluble in appropriate organic solvents or may be heterogeneous catalysts.
According to the prior art, several systems are used as catalysts in the copolymerization of epoxides and carbon dioxide. These catalysts compromise compounds or mixtures of compounds that are prepared from metal-alkyl compounds such as diethyl zinc or trimethyl aluminum (see, e.g. J. Am.Chem, Soc. 2000, 122, 12487 by Darensbrough, Wildeson, Yarbrough, Reibenspies or Polym. Mater. Sci. Eng. 1996, 74, 431, by Darensbrough). Co-reacting compounds in the preparation are, in general, compounds with Broendsted acidic properties, amongst others, phenols, alcohols, carboxylic acids (see, e.g., Polymer 2002, 43, 4535-43, Hsu, Tan), acetonates, ketoiminates (see, e.g. Angew. Chem., Int. Ed. 2002, 41(14), 2599, Moore D. R. Cheng M. Lobkovsky E B. Coates G W). Metal amides such as Zn(di(bistrimethylsilylamid) have been applied as well. Metal alkyl compounds and amides are expensive and sensitive to air and in part even self-igniting. They are therefore hard to handle on an industrial scale.
Chromium catalysts have been described, e.g. by Holmes and coworkers in: Macromolecules (2000) 33, 303. However, chromium catalyst residues are not readily removed and yield colored polymer products. Such catalysts also show high tendency to form cyclic compounds. Also, metal salts may be used as catalyst precursors, but catalysts containing these compounds still lack the catalytic activity needed for a useful production process.
In summary, activity and productivity as well as safety compatibility of the catalysts containing Zn known from the prior art are limited. Furthermore, the catalysts known from the prior art are molecular in nature and do not form a porous framework by themselves, said framework being a particularly advantageous embodiment for the realization of these reactions in industrial-scale reactors. In summary, a need exists for new catalysts to be used for these polymerization reactions if they are to be commercialized.
It would therefore be of importance to develop a solid state catalyst with a high number of accessible centers which optionally can be removed by filtration or sedimentation from the polymer product mixture. For example, one of the most active solid state catalysts, ZnGlutarates as prepared by Ree from Glutaric acid and ZnO, has a BET-surface area of less than 20 m2/g. It would obviously be of importance to develop high surface area catalysts.
In a promising novel and alternative strategy to create micro- and/or mesoporous catalytically active materials in general, metal ions and molecular organic building blocks are used to form so-called metal-organic frameworks (MOFs). The metal-organic framework materials as such are described, for example, in. U.S. Pat. No. 5,648,508, EP-A-0 709 253, M. O""Keeffe et al., J Sol. State Chem., 152 (2000) p. 3-20, H. Li et al., Nature 402 (1999) p. 276 seq., M. Eddaoudi et al., Topics in Catalysis 9 (1999) p. 105-111,B. Chen et al., Science 291 (2001) p. 1021-23.
Among the advantages of these novel materials, in particular for applications in catalysis, are the following:
(i) larger pore sizes can be realized than for the zeolites used presently
(ii) the internal surface area is larger than for porous materials used presently
(iii) pore size and/or channel structure can be tailored over a large range
(iv) the organic framework components forming the internal surface can be functionalized easily
(v) the metal-organic framework according to the invention is stable even if no host, solvent or any other additional substance is present, i.e. the framework does not collapse and/or interpenetrate and/or change its shape and dimension. This puts the material according to the invention in contrast to other metal-organic materials that maybe used as catalysts.
However, these novel porous materials have only been described as such. The use of these catalytically active materials for copolymerization reactions, in particular for the polymerization of epoxides with carbon dioxide, has not been disclosed yet. In related applications, the use of these novel porous materials as shaped bodies (U.S. application Ser. No. 10/157,182) and for epoxidation reactions (U.S. application Ser. No. 10/157,494) has been described. In the context of the present invention it is of particular interest that Zn is easily built as an active metal ion into said frameworks.
It is an object of the present invention to provide a process and a catalyst for the reaction of an alkene oxide or an alkene oxide precursor in the presence of carbon dioxide or any substance delivering carbon oxide, wherein the catalyst for said reaction contains a novel, more effective material, in addition to, or instead of, catalytic materials according to the prior art.
This object is solved by providing a process for the reaction of an alkene oxide or an alkene oxide precursor in the presence of carbon dioxide or any substance delivering carbon oxide in the presence of a catalyst, wherein said catalyst contains a metal-organic framework material comprising pores and at least one metal ion and at least one at least bidentate organic compound, which is coordinately bound to said metal ion, and wherein said framework material retains its dimension and shape even while no other materials are present.